1. Field of the Disclosure
This disclosure relates to a method of manufacturing a liquid crystal display device, and more particularly to a method of manufacturing a thin film transistor array substrate.
2. Description of the Related Art
In general, the liquid crystal display (LCD) device uses an electric field and controls a light transmittance of liquid crystal having a dielectric anisotropy, in order to display pictures. The LCD device is formed by combining a color filter array substrate and a thin film transistor array substrate in the center of liquid crystal.
The TFT array substrate includes a thin film transistor and a pixel electrode which are formed in each pixel region on a substrate. The pixel regions are defined by crossing gate lines and data lines on the substrate. The thin film transistor responds to a gate signal from the gate line and applies a data signal from the data line to the pixel electrode. The pixel electrode is formed of a transparent conduction layer. This pixel electrode depends on the data signal from the thin film transistor and drives the liquid crystal. The liquid crystal allows its molecules to rotate in accordance with an electric field, which is formed by the data signal on the pixel electrode and a common voltage on a common electrode, and controls its light transmittance, thereby implementing a gray scale. The common voltage is provided as a reference of driving the liquid crystal. The common electrode receiving such a common voltage can be formed on either the thin film transistor array substrate or the color filter array substrate. 
The thin film transistor array substrate of the LCD device is manufactured through multiple masking processes. Each masking process involves a plurality of steps including depositing a thin film, washing the thin film, forming a photoresist pattern, etching the thin film, stripping the photoresist pattern, testing the etched thin film, etc. Such multiple masking processes become the primary factor in complicating the manufacturing process of the thin film transistor array substrate and increasing the cost of the LCD device.
To address this matter described above, a four-step masking process, which uses a diffraction-light exposing mask and reduces the procedure by one step from a previous five-step masking process, has been proposed as a part of the manufacturing method of the TFT array substrate. The four-step masking process includes a first masking process for forming gate patterns, a second masking process for forming semiconductor and source/drain patterns, a third masking process for forming a protective film pattern, and a fourth masking process for forming transparent electrode patterns.
However, since the four-step masking process is also complicated, it continues to be difficult to decrease the manufacturing cost. Accordingly, a simpler manufacturing method adapted to reduce more of the manufacturing cost of the thin film transistor array substrate is required.